The present invention relates to one-component RTV silicone rubber compositions and more particularly the present invention relates to one-component silicone rubber compositions with good bonding properties even at high humidity conditions.
One-component RTV silicone rubber compositions (RTV stands for room temperature vulcanizable) are well known. Examples of such compositions are for instance disclosed in U.S. Pat. No. 3,382,205, and U.S. Pat. No. 3,296,195 which are hereby incorporated by reference. Those compositions comprise as the basic ingredient a silanol terminated diorganopolysiloxane polymer having a viscosity varying from 100 to 1,000,000 centipoise at 25.degree. C., an acyloxy functional cross-linking agent and a metal salt of a carboxylic acid as a curing promoter. The composition may contain fillers, self-bonding additives, flame-retardant additives and other ingredients that are necessary. The most common acyloxy functional cross-linking agent is methyl, triacetoxysilane. The most common and the most desirable curing promoter is a tin soap. The most common types of fillers are fumed silica or precipitated silica treated or untreated with various types of extending fillers such as crushed quartz, diatomaceous earth etc. A preferred self-bonding additive for such a composition is disclosed in Kulpa, U.S. Pat. No. 3,296,161 which is hereby incorporated by reference.
Irrespective of what ingredients are utilized, such compositions are packaged in a single package in the substantial absence of moisture and stored as such. When it is desired to cure the composition, the seal on the package is broken and the composition is applied to cross-link and cure by exposure to atmospheric moisture to a silicone elastomer with final cure taking place in a period of time varying anywhere from 24 to 72 hours. Other types of one-component RTV compositions are also known with various types of cross-linking agents such as alkoxy functional cross-linking agents such as are disclosed in Beers U.S. Pat. No. 4,100,129.
The acetoxy functional RTV systems are desirable in that they are relatively simple and inexpensive to produce. However, such compositions had one major drawback and that is that they gave off acetic acid upon curing which was corrosive in some applications and had a pungent odor. Nevertheless, such acetoxy functional RTV systems were utilized as sealants and gasket forming materials in the automotive industry as early as 1971 for various purposes. It was found that these acetoxy functional RTV systems had acceptable bonding properties to the substrates even at high humidity conditions that is, at a Relative Humidity of 80% or more. However, as stated, such acetoxy systems were corrosive and the acid that was released tended to discolor the metal to which it was applied. Since that time, the use of RTV in the automotive industry has increased and it has been desirable to find a gasket sealant for automotive applications which would have good bonding properties to the steel or metal substrate. Accordingly, it was highly desirable to find a one-component RTV system which was relatively non-corrosive and which could be utilized as a gasketing material in the automotive industry, and which further did not have a pungent odor. It should be noted that the alkoxy functional RTV of Beers U.S. Pat. No. 4,100,129 did have these properties, that is it was not corrosive and did not give off a pungent odor, but unfortunately such systems cured too slowly for automotive applications in most cases. Accordingly, it was highly desirable to develop a one-component RTV system that was relatively non-corrosive and did not give off a pungent odor and which had high temperature thermal properties and could be utilized satisfactorily as a gasketing sealant in automotive applications.
An example of such a sealant is disclosed in Beers, U.S. Pat. No. 4,257,932. Such sealant which was acyloxy functional differed from the traditional acetoxy functional RTV systems in that the acyloxy group in the cross-linking agent had from 5 to 30 carbon atoms and was most preferably methyl tris (2-ethyl hexanoxy) silane. There was incorporated into such a composition a plasticizer and an adhesion promoter which generally comprised a fluid polysiloxane having a high degree of tri-functionality or tetra-functionality as disclosed in the foregoing patent, fumed silica as the filler, a tin soap and preferably iron oxide as a thermal stablizer for the composition. It was disclosed in this patent there may be preferably incorporated in the composition as the self-bonding agent a silyl isocyanurate. Other modifications of this system are for instance, disclosed in Smith, et al., U.S. Pat. No. 4,247,445 which is hereby incorporated by reference. Another modification of this system was the utilization as disclosed in Mitchell, et al., U.S. Pat. No. 4,273,698 the incorporation therein of silyl maleates, silyl fumerates and silyl succinates as self-bonding additives. The foregoing silyl maleates, silyl fumerates and silyl succinates of the foregoing Mitchell et al. patent which is hereby incorporated by reference was preferable to the isocyanurates as adhesion promoters since they gave better bonding properties to the foregoing RTV systems of U.S. Pat. No. 4,257,932. Another modification of such a system was the pre-reacting of the cross-linking agent with the acyloxy functional silane cross-linking agent and preferably also the tin soap so as to yield more shelf stable compositions, as disclosed in Dziark et al., U.S. Pat. No. 4,308,372 which is hereby incorporated by reference. All of these patents disclose improvements over the basic composition of U.S. Pat. No. 4,257,932 which result in an improved composition. However, the improvements disclosed in the foregoing Mitchell, et al. patent and the Dziark et al. patent application were especially desirable in the application of the 2-ethyl hexanoxy functional RTV system in the automotive industry.
However, it was found that such compositions had a problem when applied as a gasketing material. At environments of up to 50% Relative Humidity, most of the time the composition had acceptable bonding properties. However, it was noted that such systems did create a brown film on the steel substrate. However, what was more noticeable was that in the area of 70, 80 and more percent Relative Humidity the liberated acid upon the cure of the 2-ethyl hexanoxy system would react with the iron in the steel to form iron octoate which would cause a brown oil deposit on the steel and which acted as a release agent destroying the self-bonding properties of the composition. Although there was some loss of adhesion with the acetoxy systems, it was much more noticeable with 2-ethyl hexanoxy RTV systems.
Accordingly, it was highly desirable to find a scavenger which would lock up the acid as it was released in the RTV system such that the self-bonding properties of the system would be maintained. It should be noted that self-bonding properties are highly desirable in RTV systems, otherwise a primer has to be utilized which requires additional material and labor; thus increasing the cost of producing the silicone gasket. Accordingly, it was highly desirable to solve this problem so that the 2-ethyl hexanoxy RTV system would have good self-bonding properties to metal substrates, and other substrates even at high percent Relative Humidity of 70, 80 or more.
It is one object of the present invention to provide for a hexanoxy functional RTV system with good self-bonding properties even at high percent Relative Humidity.
It is yet another object of the present invention to provide for a hexanoxy functional RTV system that does not discolor the underlying metal to which it is applied and which has good self-bonding properties to the underlying metal substrate.
It is yet an additional object of the present invention to provide for an acid scavenger for acyloxy functional RTV systems so as to maintain the self-bonding properties of the composition even at high percent relative humidity.
It is yet also an additional object of the present invention to provide a process for forming a silicone gasket in the automotive industry from an acyloxy functional RTV system in which the system contains an acid scavenger.
It is yet still an additional object of the present invention to provide a process for forming a silicone gasket against a metal substrate for automotive applications in which the silicone gasket has good self-bonding properties even at elevated percent Relative Humidity, is relatively non-corrosive, does not discolor the metallic substrate and does not have a pungent odor.
It is still an additional object of the present invention to provide for an improved silicone gasket for the automotive industry which has good self-bonding properties to metallic substrates.
These and other objects of the present invention are accomplished by means of the disclosures set forth here below.